This is a plug that fits the 30mm diameter hole in the back of the CR5 enclosure. If you are not using the spool holder it seems like a good idea to plug the hole to keep hot air inside. It fits tight. I would only print this with TPU as the material needs some flexibility to squeeze through the opening. The inner diameter is 30mm and the outside is 34mm. Its basically an Oreo! I was a little worried about the overhang but it printed fine without supports.